This invention relates to energy storage devices and, more particularly, to supercapacitors using conducting polymers for charge storage. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
A promising application of conducting polymers is in energy storage devices. Battery systems have been extensively studied where a p-dopable conducting polymer is used as the cathode active material and a metal, often lithium, is used as the anode active material. Various p-dopable materials have been studied: polyacetylene, poly-(p-phenylene), polypyrrole, polyaniline, etc. A battery with n-dopable conducting polymer, polyacetylene, as the anode material and a metal oxide as cathode active material has also been described.
The nature of charge storage within conducting polymers is complex and has been treated as being a mixture of Faradaic (battery type) and capacitive (not battery type) components. The electrode potential during the discharge of a battery electrode is expected to remain ideally constant and hence the cell voltage of a battery remains fixed until it goes abruptly to zero when the relevant chemicals are consumed. On the other hand, the potential at an electrode when a capacitive reaction takes place ideally changes linearly with charge, in exactly the same way as the voltage across a regular capacitor, by Q=CV.
In general, this is the type of behavior observed in the discharge of doped conducting polymers. The reason for observed capacitive behavior in electrochemical systems is precisely that a capacitor is formed at an electrode/electrolyte interface by electronic charging of the solid electrode material (e.g., a metal or carbon) with counter-ions in the solution phase migrating to the electrode in order to balance that charge. To some degree, this is what occurs in conducting polymer electrodes, but the charging process occurs through the volume of the active polymer material rather than just at the interface. When the conducting polymer is being p-doped, electrons leave the polymer backbone, generating an excess of positive charge. To counter this charge, anions from the electrolyte solution migrate through the film and position themselves adjacent to the positively charged polymeric units. In the case of n-doping of conducting polymers, the reverse process takes place in that the polymer backbone becomes negatively charged by the addition of electrons from the external circuit and cations enter the film from the solution to balance this charge.
In a typical electrochemical capacitor, the distance that separates the charges is nominally in the order of angstroms and the capacitances that can be obtained are orders of magnitude higher than in conventional capacitors, including electrolytic capacitors. The advantage of electrochemical capacitors over batteries is, that while typical energy densities are lower, the available power densities are much higher. The reason for this is that no phase transformation takes place when a capacitive electrode is charged or discharged, only the movement of electronic charge through the solid electrode phase and ionic movement through the solution phase. In a typical battery reaction, metal dissolution, for example, there are constraints on the maximum rate of the electrochemical reaction, and, hence, power density, that are associated with the requirement for phase transformation.
It has been suggested that possible applications for electrochemical capacitors include energy sources for computer memory back-up and high power sources for electric vehicles. Capacitor configurations using conducting polymers have been previously discussed where the two electrodes contain equal amounts of the same p-dopable conducting polymer (a Type I configuration as herein discussed) or one electrode is a p-dopable conducting polymer electrode and the other is a high surface area carbon electrode.
The present invention will consider three types of electrode configurations forming a unit cell in a capacitor:
Type I--both electrodes contain the same amount of the same p-dopable conducting polymer; PA0 Type II--two different p-dopable conducting polymers form the electrodes, where the voltage range over which one of the conducting polymers is oxidized from neutral to p-doped is substantially different from the range for the other polymer; PA0 Type III--each conducting polymer is in its conducting doped state when the capacitor is fully charged, one n-doped and one p-doped. Prior art capacitors have been limited to Type I configurations. Prior art batteries have been reported with Type II and Type III configurations. Type III batteries require a polymer than can be both n-doped and p-doped, such as polyacetylene, poly-(p-phenylene), and polythiophene and derivatives. It is difficult to n-dope polythiophene and prior art Type III batteries have used polyacetylene and poly-(p-phenylene) electrodes in a symmetric configuration and polyacetylene/polypyrrole in an unsymmetric configuration. Only low power applications, e.g., batteries, have been considered, however, because unfavorable material properties of the active conducting polymers preclude high power applications.
It would be desirable to provide high energy density capacitors using conducting polymers. However, such devices are preferably formed of polymers that can be formed in a relatively thick layer for high volume energy storage. Ion transport through the polymer must be rapid to provide a high output power. The present invention is directed to these characteristics and it is an object of the present invention to provide a supercapacitor, i.e., a high output energy capacitor using conducting polymers.
It is another object of the present invention to provide an electrolyte that enables rapid ion transport through the active conducting polymer film.
One other object of the present invention is to maximize the stored energy that can be delivered from a capacitor.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.